In high-frequency circuit sections such as an RF stage in a transmitting circuit and a receiving circuit for mobile communication devices or the like including analog or digital mobile phones or wireless phones, for example, in the case where the same antenna is shared by the transmitting circuit and the receiving circuit, in order to remove unnecessary signal waves other than desired signal waves such as to separate a transmission frequency band and a reception frequency band or to attenuate a harmonic generated based on non-linearity of an amplifier circuit, band pass filters are frequently used. Such band pass filters used for communication devices are mostly constituted by microstriplines or the like because filter circuits sections can be small or electrical characteristics as high-frequency circuits are satisfactory.
The band pass filters which are constituted by the microstriplines can be easily applied to MIC (Microwave Integrated Circuits) and MMIC (Monolithic Microwave Integrated Circuits), but band pass filters using conventional microstriplines are constituted by a plurality of ¼ wavelength (hereinafter, means electrical length) lines which are side-coupled.
In general, two representative characteristics are known as the characteristics of the band pass filters. One is a Chebyshev characteristic shown in FIG. 8A, and a ripple appears in passband, but cut-off characteristic (steepness) is satisfactory. The other one is a Butterworth characteristic shown in FIG. 8B, and since the passband is flat and a ripple is less, this is suitable for accurate measurement. In FIGS. 8A and 8B, a solid line expresses a pass characteristics (amplitude), a broken line expresses a group delay characteristics.
FIG. 4 is a diagram illustrating an example of a band pass filter in which eight stages of conventional ¼ wavelength (λ/4) lines are side-coupled, and it is a Chebyshev type filter. FIG. 5 is a diagram illustrating its high-frequency characteristics, and in this example, insertion loss at 2 GHz is 0.8059 dB, a group delay time is 2.4585 ns, and the fractional bandwidth (3 dB pass-bandwidth/pass center frequency) is about 45%. Since the fractional bandwidth of a one-stage band pass filter which is constituted by a ¼ wavelength line is generally about 15%, a number of the stages in this example is set to eight in order to extend the band, but on the contrary, a circuit is enlarged, thereby increasing insertion loss. Further, in the Chebyshev type filters, when the passband is made to be flat, the group delay characteristics do not become constant, and thus the waveform is easily distorted.
FIG. 6 is diagram illustrating an example of a band pass filter in which six stages of conventional ¼ wavelength (λ/4) lines are side-coupled, and it is a Butterworth type filter. FIG. 7 illustrates its high-frequency characteristics, and in this example, insertion loss at 2 GHz is 0.664 dB, a group delay time is 1.9995 ns, and the fractional bandwidth is about 32%. In order to obtain cut-off characteristics which are as steep as possible by enlarging the fractional bandwidth, the number of stages is six, but for this reason, the circuit size is increased, and the insertion loss increases. The steepness on the cut-off band is inferior to that in the Chebyshev type, but the group delay characteristics are satisfactory and are approximately constant in the passband, and thus the waveform is hardly distorted. In the band pass filter which is constituted by the conventional microstriplines, since the resonance frequency is determined by ¼ wavelength, it is difficult to extend the band (about 15%). Further, when a number of stages is increased in order to extend the fractional bandwidth, the circuit size is increased and the insertion loss increases, and thus this filter is not suitable for MIC and MMIC.
Further, in order to remove disadvantages that a shape of the band pass filters in which a plurality of conventional ¼ wavelength lines are side-coupled is large and the insertion loss is large, a dual-mode filter which uses a ring resonator is known (see Japanese Patent Application Laid-Open No. 9-139612). This filter is small, but it has an essential problem such that the band is narrow. That is to say, in the conventional filters using the ring resonators, since impedance becomes minimum at the resonance frequency, only the resonance portion passes, the other band portions are rejected. Due to its properties, therefore, the passband must be narrow.
On the other hand, a band rejection filter which does not allow only a signal at a specified frequency to pass and allows signals at the other frequencies to pass is known, but this band rejection filter does not allow only signals at a specified frequency (attenuation pole frequency) and at frequencies within a narrow range before and after the specified frequency to pass, and allows signals at the other frequencies to pass. For this reason, when this filter is used as the band pass filter, it can be a wideband band pass filter. In the band rejection filter, however, since a frequency band which rejects the passing is narrow, it has a problem such that it also allows signals which are not desired to be passed to pass. Particularly, this filter cannot be used for the case where a DC component should be removed.
Conventionally-known filters that reject the DC component include a filter that uses a ¼ wavelength short stub shown in FIG. 13. This filter can remove the DC (and frequency which is two times as high as pass center frequency) component as shown in FIG. 14, but reflection frequently occurs at frequencies other than the pass center frequency (see S11), and the loss is large. A filter which rejects the DC component and has less reflection (loss) at the passband is, therefore, desired. FIG. 14A illustrates a simulation result, and FIG. 14B illustrates actual measurement data.
The present invention is devised in order to solve the problems of the conventional band pass filter and band rejection filter, and its object is to provide a filter in which insertion loss is small in wideband, a passband is flat, steep attenuation is obtained and a DC component can be removed, and a high-frequency band pass filter utilizing this filter.